Magnetism - Part IV

3. Theory of magnetism
We have seen in the earlier section how a current carrying conductor produces a magnetic field.  An atom has negatively charged electrons orbiting around the positively charged nucleus. Each circulating electron can be visualized as a current loop. Thus electrons produce magnetic fields around themselves. In an atom, a pair of electrons in the same orbit, revolve in clockwise and anti-clockwise directions. This cancels out the magnetic field produced by one electron with that produced by its pair. Only unpaired electrons are able to produce magnetic fields. Larger the number of unpaired of electrons, larger will be the magnetic field surrounding the atoms. As seen in case of solenoids, the magnetic field generated by an orbiting electron is along the axis of the orbit. This is generally shown by an arrow.

In substances like copper, bismuth, the atomic magnetic field is non-existent because of paired electrons.  These are called non-magnetic substances. In substances like aluminum, platinum, there are unpaired electrons; they exhibit weak magnetic fields. At ordinary temperatures, the magnetic directions of atoms are random. Hence these materials show non-magnetic behaviour at ordinary temperatures. If an external magnetic field is applied, the atomic magnets get aligned with respect to the direction of the applied magnetic field. This behaviour is enhanced at low temperatures as the random motion of the atoms is reduced. Thus the substances which show magnetic behaviour on application of a magnetic field are called paramagnetic substances. When the field is removed, these substances show no magnetic properties.

On the other hand substances like iron, nickel and cobalt show very high degree of alignment and magnetic behaviour with the application of external magnetic field. Even when the external magnetic field is removed, these substances continue and retain all magnetic properties. These substances are called ferromagnetic materials. Ferromagnetic properties may be lost at high temperatures, as random alignment of atoms is increased.  

Thus electron orbit alignments or atomic alignments is the core to the magnetic behaviour of a material.  If by any method, the atomic alignment is disturbed or is made random, the material will loose its magnetism. This is the reason why breaking or heating a magnet destroys magnetism, as mentioned earlier. Also we have seen that if you break a magnet, the north and south poles will form immediately in the smaller pieces. The reason for this phenomenon again lies in the atomic alignments. At the microscopic level in a magnetic material, the atoms with unpaired electrons will align themselves, however small divisions you make.

4. Earthís magnetic field

The earth itself acts like a huge magnet. It is not known exactly why the earth is a magnet, it is not a magnetized chunk of iron like a bar magnet, and is too hot for individual atoms to remain aligned.  It is presumed that since the earthís core is made of molten metal such as iron or nickel, which are already ferromagnetic materials, they are responsible for the earthís magnetic field.  The Sun also has a similar magnetic field. Such fields are known as geomagnetic fields. It is found that earthís geomagnetic field reverses direction every 70,000 years! It is still a mystery why this occurs. Extinction of animals such as the dinosaurs is attributed to a sudden reversal of earthís magnetic field. Animals such as birds and insects are sensitive to earthís geomagnetic field and rely on their in-built compasses to find directions.

The strength of earthís magnetic filed is about 0.7 gauss. When a bar magnet is held freely it points in the north-south direction. But it is seen that earthís magnetic poles are not coinciding with the earthís geographical axis.  The magnetic south pole is more than 1800 kilometers east of the actual geographical north pole.

Earthís magnetic field protects us from the harmful charged particles that reach the earth from the sun. These charged particles are deflected away from the earth by the earthís magnetic lines of force.  At any point on the surface of the earth, the geomagnetic field is taken as Hcos, where H = 0.7 gauss and is the angle of the local latitude. The earthís magnetic field is taken as zero at the poles and is maximum at the equator.

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